1. Field of the Invention
The present invention relates to an active muffler that muffles noise by generating a sound having opposite phase to that of the noise, particularly to a muffler having improved response characteristics.
2. Description of Related Art
A muffler that actively muffles noise by generating a sound having opposite phase to that of the noise has been used since long time ago.
FIG. 1-1 is a view schematically showing how noise is muffled by an active muffler. As shown in FIG. 1-1, in order to cancel out the noise coming from a noise source 10 at a place where a person 50 is present, the active muffler picks up the noise from the noise source with a microphone 20, amplifies the noise signal in opposite phase with an amplifier 30, and generates a sound having opposite phase with a speaker 40.
FIG. 1-2 shows a concrete configuration example for actively muffling noise. The noise is converted into an electrical signal by a microphone A20, the electrical signal is processed by an adaptive filter 32 so that a sound suitable to muffle the noise is generated when being played by the speaker 40, and the signal processed by the adaptive filter 32 is amplified by the amplifier 30 and then outputted by the speaker 40. The outputted sound cancels out the noise, and a monitoring microphone B34 detects whether or not the noise has been suitably muffled. An electrical signal converted by the monitoring microphone B34 is fed back to the adaptive filter 32 where a coefficient of the adaptive filter 32 is changed so that a suitable sound can be generated by the speaker 40.
These configurations are mostly achieved by converting the inputted electrical signal into a digital signal, and performing digital signal processing on the digital signal by using a DSP (digital signal processor). Refer to, for example, Patent Document 1 for details of the active muffler.
One of the problems with the use of the speaker of the muffler is response lag caused by the speaker, as indicated by graphs of FIGS. 2A and 2B. FIG. 2A is a graph of input signal to the speaker, and FIG. 2A is a graph indicating the movement of the speaker.
As indicated by the graphs of FIGS. 2A and 2B, in the case where a step input signal shown in FIG. 2A is applied to an ordinary dynamic speaker having a voice coil, the speaker will cause an operating delay on rising edge as shown in FIG. 2B. When such operating delay is caused, it will not be possible to sufficiently perform sound-muffling at the moment when the sound-muffling operation is started if the distance between the speaker and the sound-muffling area is small.
Further, in the case where noise is generated from a flat surface (for example, a floor of an upstairs room of an apartment building), a flat speaker having a flat diaphragm and capable of generating a plane wave may be used to cancel out the noise. A case where a flat speaker is used to cancel out such noise will be described below with reference to FIG. 3. In FIG. 3, noise is generated from a flat surface 12. Since the noise is generated from the flat surface 12, the noise propagates through air as a plane wave. On the other hand, when a plane wave having opposite phase to that of the noise is generated from a flat speaker 50, the wave crest (+) and the wave trough (−) of the plane wave of the noise and the wave crest (+) and the wave trough (−) of the plane wave of the generated sound will coincide with each other and therefore completely cancel out each other, so that the noise is muffled.
In the case where a flat speaker is used to cancel out the noise generated from a large flat surface, it is necessary to drive a flat diaphragm using a plurality of voice coils. However, due to variation in characteristics of the plurality of the voice coils, the flat diaphragm can not be uniformly driven, and that is a problem.
Refer to, for example, Patent Document 2 for details of a configuration in which a flat speaker is used to actively muffle noise.    Patent Document 1: Japanese Unexamined Patent Application Publication No. Hei 5-61480    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2007-321332